1. Field of the Invention
The invention relates generally to down hole tools having fluid with an adjustable viscosity, and, more specifically, to down hole tools using magnetic fields to adjust the viscosity of a magnetorheological fluid.
2. Background Art
Many down hole tools used in drilling for hydrocarbons and for servicing existing wellbores to aid hydrocarbon recovery include oil or other viscous fluid to control actuation of the down hole tool. One such down hole tool is a jar.
Jars are used during drilling and fishing operations to free other components of a drill string that have gotten stuck in the wellbore. Jars provide an impact blow in the up or down directions. A driller can control the jarring direction, impact intensity, and jarring times from the rig floor. The magnitude and direction of the load used to initiate the impact blow achieve this control.
FIGS. 1A and 1B show cross sections through a detent portion 11 of a prior art jar 10. To jar upward, the drill string is pulled in tension. Upward force arrow 13 is shown applied to a mandrel 12 of the jar 10. This force is transmitted to outer cylindrical housing 14 by a resulting increase in pressure in the hydraulic fluid that is contained in the upper chamber 16 between the outer cylindrical housing 14 and the mandrel 12.
The magnitude of pressure in the upper chamber 16 is directly proportional to the magnitude of the force applied to the mandrel 12 by pulling the drill string upward. This high-pressure fluid is allowed to flow through an orifice 18 to a lower chamber 20. A check valve (not shown) is sometimes used to restrict the flow through the orifice 18. The result of this fluid flow is a relative axial movement between the outer housing 14 and the mandrel 12. This axial movement occurs slowly until the orifice 18 is in juxtaposition to a relief area 17 of outer housing 14, which causes a sudden release of high pressure to occur as the fluid flow is no longer restricted by the orifice 18. The sudden release of high pressure results in an impact below being delivered to the “knocker” part (not shown) of the jar as the tension in the drill string is released. The knocker is normally located at the upper most end of the jar.
During the restricted flow of the hydraulic fluid, the temperature increases as a result of the high pressures and friction through the orifice 18. Temperature also increases from being down hole, which is typically much hotter than surface temperatures. The increasing temperature reduces the viscosity of the hydraulic fluid, which allows flow through the orifice 18 to occur faster. The faster flow reduces the amount of time it takes to fire off the jar. With successive firings, the viscosity can be reduced so much that there is insufficient time to pull the drill string in tension before the jar fires again. This causes successive jar firings to be weaker each time until becoming completely ineffective.
To compensate for the increased temperature down hole, the viscosity of the hydraulic fluid may be made to be higher during assembly of the jar. The process of adjusting viscosity at the surface typically involves mixing oils of different viscosity. The selected viscosity is also based on the desired timing for the jar between initial loading and firing. Regardless of adjustments made at the surface, each successive firing of the jar will continue to warm up the hydraulic fluid until the viscosity decreases so much that the jar becomes ineffective. In many cases, the jarring operation will successfully unstick the drill string before the hydraulic fluid gets too hot, but in other cases the jarring operation will have to be stopped before success is achieved in order to replace or rebuild the jar.
This issue is not limited to jars, but rather may occur with any tool that interacts with hydraulic fluid, such as downhole shock (absorber) tools, accelerators, and other tools known to those of ordinary skill in the art.